qt wizard bip39 recovery: better handle --offline mode

```
 32.40 | E | gui.qt.exception_window.Exception_Hook | exception caught by crash reporter
Traceback (most recent call last):
  File "/home/user/wspace/electrum/electrum/gui/qt/wizard/wallet.py", line 709, in <lambda>
    button.clicked.connect(lambda: Bip39RecoveryDialog(self, get_account_xpub, on_account_select))
  File "/home/user/wspace/electrum/electrum/gui/qt/bip39_recovery_dialog.py", line 40, in __init__
    fut = asyncio.run_coroutine_threadsafe(coro, network.asyncio_loop)
AttributeError: 'NoneType' object has no attribute 'asyncio_loop'
```

electrum/bip39_recovery.py

@@ -2,7 +2,7 @@
 # Distributed under the MIT software license, see the accompanying
 # file LICENCE or http://www.opensource.org/licenses/mit-license.php
 
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Optional
 import itertools
 
 from . import bitcoin
@@ -10,13 +10,15 @@ from .constants import BIP39_WALLET_FORMATS
 from .bip32 import BIP32_PRIME, BIP32Node
 from .bip32 import convert_bip32_strpath_to_intpath as bip32_str_to_ints
 from .bip32 import convert_bip32_intpath_to_strpath as bip32_ints_to_str
-from .util import OldTaskGroup
+from .util import OldTaskGroup, NetworkOfflineException
 
 if TYPE_CHECKING:
     from .network import Network
 
 
-async def account_discovery(network: 'Network', get_account_xpub):
+async def account_discovery(network: Optional['Network'], get_account_xpub):
+    if network is None:
+        raise NetworkOfflineException()
     async with OldTaskGroup() as group:
         account_scan_tasks = []
         for wallet_format in BIP39_WALLET_FORMATS:

electrum/gui/qml/qebip39recovery.py

@@ -9,6 +9,7 @@ from electrum import Network, keystore
 from electrum.bip32 import BIP32Node
 from electrum.bip39_recovery import account_discovery
 from electrum.logging import get_logger
+from electrum.util import get_asyncio_loop
 
 from .util import TaskThread
 
@@ -84,7 +85,7 @@ class QEBip39RecoveryListModel(QAbstractListModel):
         network = Network.get_instance()
         coro = account_discovery(network, self.get_account_xpub)
         self.state = QEBip39RecoveryListModel.State.Scanning
-        fut = asyncio.run_coroutine_threadsafe(coro, network.asyncio_loop)
+        fut = asyncio.run_coroutine_threadsafe(coro, get_asyncio_loop())
         self._thread.add(
             fut.result,
             on_success=self.on_recovery_success,

electrum/gui/qt/bip39_recovery_dialog.py

@@ -12,6 +12,7 @@ from electrum.i18n import _
 from electrum.network import Network
 from electrum.bip39_recovery import account_discovery
 from electrum.logging import get_logger
+from electrum.util import get_asyncio_loop, UserFacingException
 
 from .util import WindowModalDialog, MessageBoxMixin, TaskThread, Buttons, CancelButton, OkButton
 
@@ -37,7 +38,7 @@ class Bip39RecoveryDialog(WindowModalDialog):
         self.thread.finished.connect(self.deleteLater) # see #3956
         network = Network.get_instance()
         coro = account_discovery(network, self.get_account_xpub)
-        fut = asyncio.run_coroutine_threadsafe(coro, network.asyncio_loop)
+        fut = asyncio.run_coroutine_threadsafe(coro, get_asyncio_loop())
         self.thread.add(
             fut.result,
             on_success=self.on_recovery_success,
@@ -81,8 +82,12 @@ class Bip39RecoveryDialog(WindowModalDialog):
         if isinstance(e, concurrent.futures.CancelledError):
             return
         self.clear_content()
-        self.content.addWidget(QLabel(_('Error: Account discovery failed.')))
-        _logger.error(f"recovery error", exc_info=exc_info)
+        msg = _('Error: Account discovery failed.')
+        if isinstance(e, UserFacingException):
+            msg += f"\n{e}"
+        else:
+            _logger.error(f"recovery error", exc_info=exc_info)
+        self.content.addWidget(QLabel(msg))
 
     def clear_content(self):
         for i in reversed(range(self.content.count())):

electrum/network.py

@@ -407,6 +407,9 @@ class Network(Logger, NetworkRetryManager[ServerAddr]):
 
     @staticmethod
     def get_instance() -> Optional["Network"]:
+        """Return the global singleton network instance.
+        Note that this can return None! If we are run with the --offline flag, there is no network.
+        """
         return _INSTANCE
 
     def with_recent_servers_lock(func):

electrum/util.py

@@ -204,6 +204,14 @@ class UserFacingException(Exception):
 class InvoiceError(UserFacingException): pass
 
 
+class NetworkOfflineException(UserFacingException):
+    """Can be raised if we are running in offline mode (--offline flag)
+    and the user requests an operation that requires the network.
+    """
+    def __str__(self):
+        return _("You are offline.")
+
+
 # Throw this exception to unwind the stack like when an error occurs.
 # However unlike other exceptions the user won't be informed.
 class UserCancelled(Exception):